Method of growing semiconductor crystals



Nov. 22, 1960 w. c. DASH METHOD OF GROWING SEMICONDUCTOR CRYSTALS Filed Dec. 27, 1957 To Vcuurn Pump m w m A Wm WJ METHQD F GROWING SEMICONDUCTOR CRYSTALS William Q. Dash, Ballston Lake, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 27, 1957, Ser. No. 705,685

Claims. (Cl. 23-301) The present invention is related to the growth of semiconductor crystals of a high degree of perfection. More particularly, the invention relates to a method of growing high purity, oxygen-free dislocation-free single crystals of silicon.

Methods for the preparation of single crystals of semiconductors such as germanium and silicon have long been known to those skilled in the art. All of these methods, however, require the growth of a single crystal by seed crystal withdrawal from a melt contained in a reasonably non-reactive crucible. The crucibles used are generally made from quartz, silicon dioxide. Although quartz is reasonably non-reactive with semi-conductor materials in the liquid state, at the high temperature involved it has been found that the semi-conductor material suifers some oxygen contamination from the quartz crucible. This is particularly true in the case of silicon which must be heated to a temperature of at least 1420 C. to be melted. Although some oxygen contamination may be tolerated, in certain instances this contamination is highly objectionable. Thus, for example, in silicon bodies, the presence of oxygen as an impurity is responsible for the phenomenon of heat treating. This phenomenon, for example, may result in a high resistivity P-type silicon crystal grown with seed rotation reverting to N-type silicon when heated to a temperature of 450 C.

Methods have been devised for growing single crystals of semiconductive material without a quartz crucible and hence avoiding oxygen contamination. These methods, however, result in a high density of dislocations within the crystal. crystal may be defined as a highly strained inner region within the crystal produced by a displacement of one large portion of the crystal lattice structure with respect to another such portion. When a semiconductor single crystal possesses a high density of dislocations, the lifetime of minority conduction carriers therein is greatly reduced and the usefulness of the crystal for the fabrication of semiconductor devices, such as transistors, is greatly reduced.

Accordingly, an object of the present invention is to provide methods for growing single crystals of substantially' oxygen-free, dislocation-free semiconductor materials.

Briefly, stated in accord with one feature of the present invention a first body of a semiconductive material capable of being heated by eddy currents is physically supported by a second body of the same material having a physical geometry which prevent the formation of eddy currents by coupling with a radio frequency electromagnetic field. Both bodies are subjected to such an electromagnetic field, resulting in the formation of a supported puddle of the semiconductive material. A monocrystalline ingot of the semiconductor material is then grown from the puddle utilizing a seed crystal having physical dimensions which are very small with respect to the amount of molten semiconductor.

The novel features characteristic of the invention are As used herein a dislocation in a single States Patent a 17. Slotted semiconductive member 18 is maintained set forth in the appended claims. The invention itself, together with further advantages and objects thereof may best be understood by reference to the following description taken in connection with the appended drawing in which:

Fig. l is a schematic illustration of one apparatus with which the invention may be practiced, and

Fig. 2 is a perspective view of a part of the apparatus of Fig. l,

I have discovered that dislocation-free, oxygen-free ingots of a semiconductor material, as, for example, germanium or silicon, may be grown by seed crystal withdrawal from a molten mass of the semiconductor suspended by levitation and surface tension atop a solid mass of the same semiconductor. Previously, this has not been possible because of the difficulty of limiting the amount of the solid semiconductor which is coupled to the electromagnetic field and heated to the melting point. I overcome this obstacle by forming a first portion of the semiconductor initially melted from a solid mass which is susceptible to induction heating or, in other words will sustain eddy currents. The support member, which may be later melted and recrystallized in the growing ingot possesses a physical geometry which renders it unable to sustain eddy currents. In a preferred embodiment the second body is cut longitudinally into sectors which are electrically disconnected circumferentially. No eddy currents can flow in the support member which is therefore not melted by induction heating, but only by thermal contact with the already molten semiconductor. The absence of dislocations from crystals grown in accord with the present invention is insured by the use of a seed crystal, the crosssectional area of which is very small with respect to the amount of molten semiconductor into which it is inserted. This minimizes the thermal strain introduced by the temperature differential between the seed and the molten semiconductor. In addition, with proper choice of crystallographic orientation of the seed, such dislocations as may be present are readily cast off in the early stages of growth.

In Fig. 1, an apparatus with which the invention may be practiced includes a crystal furnace enclosure 1, having an exhaust tube 2 for connection with a vacuum system (not shown), an induction heating coil 4 supplied with suitable radio frequency alternating electrical current from an alternating current source 6 and adjustable to move vertically about furnace enclosure 1 by means of gear train 7 mounted upon threaded vertical rod 8, driven by suitable means (not shown).

The furnace enclosure 1 is supported from the lower surface of a table top or plate 9 and secured thereto in vacuum tight relation by means of a flange and gasket assembly 10. The plate 9 is apertured centrally of the furnace enclosure to receive a crystal pulling mandril 11 which extends axially within the furnace. The mandril 11 is maintained in vacuum tight relation with the plate 9 by means of gaskets or 0 rings 12 received in receiver 13. The lower end of the mandril terminates in chuck 14, preferably of quartz, for holding a seed crystal 15 therein. In practicing theinvention a dislocation-free, oxygenfree crystal 16 of a semiconductive material is grown by I seed crystal withdrawal from a molten mass 17 of semiconductive material physically supported within the radio frequency alternating electromagnetic field generated by coil 4. The molten mass of semiconductive material 17 is supported upon a slotted cylindrical member 18 of the same semiconductive material as comprises molten mass centrally within furnace enclosure 1 by a suitable jig 19 which rests concentrically within the bottom of furnace enclosure 1.

the volume of molten semiconductor.

is shown in Fig. 2 of the drawing. In Fig. 2 support member 18 has cut therein a plurality 'of slots which extend therethrough completely separating the largest portion thereof into 60 sectors having no lateral communication with one another. Although 60 sectors are chosen by way of illustration it is apparent that any equivalent means of slotting support member 18 so that eddy currents may not flow therein thus preventing the body being coupled with the electromagnetic field to cause induction heating thereof would be suitable and sufiicient. In Fig. 2 of the drawing there is also illustrated in perspective a disc 20 of semiconductive material from which the molten mass of semiconductive material 17 in Fig. 1 of the drawing is formed. Disc 20 is conveniently cut to the same diameter as support member 18 and is, of course, of the same semiconductive material. The thickness of disc 20 is determined as follows. The upper limit of thickness is such that the molten mass 17 of semiconductive material formed by the melting thereof within the field of induction heater 4 is not sufficient so that molten semiconductive material escapes or overflows from the mass 17 atop support member 18. The lower limit of thickness is such that the molten mass 17 formed from disc 20 shall cover the entire top of support member 18. In other words, the entire mass of molten semiconductive material 17 is held in place by surface tension and electromagnetic levitation and the thickness of disc 20 is chosen to allow this condition to exist. Obviously, the amount of semiconductive material 20 will depend upon the semiconductive material chosen, and the diameter of support member 18 which governs the amount of semiconductive material which can be maintained atop support member 18 by surface tension. Thus, for example, when the semiconductive material chosen is silicon and support member and disc 20 have a diameter of /2 inch, disc 20 may conveniently be from 1 inches to inches thick.

In the practice of the invention, support member 18 and disc 20 both of the same semiconductive material are cut from suitable semiconductor bodies. While it is not necessary, both these members may conveniently be cut from a monocrystalline ingot of semiconductive material such a silicon or germanium thus insuring a high degree of purity in the final grown ingot. It is entirely feasible, however, that both support member 18 and semiconductive disc 20 may be of polycrystalline semiconductive material. Support member 18 is secured in place in jig 19 at the bottom of furnace enclosure 1. Disc 20 of semiconductive material is placed atop support member 18 and inserted into the furnace enclosure 1 and the air therein is exhausted by means of the vacuum pump (not shown) to a suitably low pressure. The temperature of semiconductive disc 20 is raised as, for example, by applying an acetylene torch to the exterior of enclosure 1 in the vicinity thereof. This is done to increase the conductivity of semiconductive disc 20 so that it may be coupled with a magnetic field and heated by induction. After the temperature of semiconductive disc 20 has been raised to a suitable temperature as, for example, 300 C. for germanium and 500 C. for silicon, electrical alternating current at radio frequencies is supplied to induction heating coil 4 which is then juxtaposed adjacent semiconductive disc 20. When semiconductive disc 20 has become molten, forming a surface tension-held molten mass 17 of semiconductive material, crystal pulling mandril 11 having seed crystal 15 therein is lowered until the seed crystal makes contact with molten mass 17. The seed crystal is inserted slightly into a molten mass until the seed crystal is observed to melt. The crystal pulling mandril is then connected to a suitable automatic feed and is slowly withdrawn.

In accord with the present invention, the grown ingot is rendered dislocation free by growth upon a seed crystal, the volume of which is extremely small as compared with In fact the seed crystal is made in the form of a thin needle just sufi'icient to support the weight of the grown ingot. This is to reduce to an absolute minimum the thermal stresses intro duced into the crystal by the difierence in temperature between the seed crystal and the molten mass of semiconductive material. In the case of silicon, dislocation-free crystals have been grown utilizing crystals having a minimum cross-sectional area of 0.0001 square inch for each 3 grams of crystal to be grown, to a maximum cross-sectional area of 0.0025 square inch.

Another way in which dislocations are minimized is by properly orienting the seed crystal. When the seed crystal is cut with the (1,0,0) plane perpendicular to the seed axis, in the growing ingot, the preferential orientation of dislocations is at an angle to the axis of the crystal ingot. In this manner, most of any dislocations which do occur, are propagated to the edge of the crystal ingot, and the remainder of the ingot is free of dislocations.

As the mandril and seed crystal are withdrawn a monocrystalline ingot of the semiconductive material nucleates upon the seed crystal to form an ingot 16 which is grown from molten mass of semiconductive material 17 at a constant rate. While I prefer that a crystal be with drawn at a rate approximately 5 mm. per minute, pulling rates of from 1 to 10 mm. per minute are suitable for the practice of the invention.

In the practice of the present invention, as the ingot 16 is withdrawn from molten mass 17 and the liquid solid interface falls the induction heating coil gradually and progressively lowered to maintain molten mass 17 at an approximately constant size by contact melting the adjacent portion of support member 18. This may conveniently be done through threaded rod 8 and gear train 7 which are conventional and are illustrated schematically in the drawing. This lowering of the induction heating coil may be done manually or may be done automatically by a suitable gear mechanism, geared with the control which automatically withdraws mandril 11 from enclosure 1 at a predetermined rate. As coil 4 is lowered, the electrical power supplied thereto is controlled to supply greater heat to molten mass 17 than is necessary to keep mass 17 molten. This causes the upper portion of support member 18 to melt, thus continually replenishing molten mass 17. It is important to note, however, that support member 18 is melted by being in contact with molten mass 17 of semiconductive material which melting is strictly controllable by varying the current supplied to and the position of induction heating coil 4. This is opposed to the condition wherein support member 18, in the absence of slotted ribs 21, would be electromagnetically coupled with the induction heating coil and would become uncontrollably heated.

In the described fashion, a monocrystalline ingot of semiconductive material is continually withdrawn from molten mass 17 of semiconductive material which is continuously replenished from support member 18. This process is continued until support member 18 is exhausted down to the point where contamination from quartz jig 19 may occur.

Since semiconductor ingot 16 is grown from a mass of semiconductive material 17 which is supported by a solid semiconductive support member 18 and by levitation from the electromangetic field the ingot is uncontaminated by contact with quartz bodies which contain oxygen and is therefore free of any objectionable oxygen contamination. Tests of wafers of silicon cut from a silicon ingot grown as described hereinbefore and subjected to infrared absorption tests at 9 microns wavelength show that there is no detectable presence of oxygen therein. Since the semiconductive ingot is grown by nucleation upon a seed crystal having a diameter which is very smallwith respect to the diameter of the grown crystal and with respect to the mass of molten material with which it is in contact initially, the disclocation density withinthe ingot is very low. Thus, 'for example, in crystals grown in accord with this process there are no detectable dislocations over a substantial portion of the crystal which comprises better than half of the central section thereof. Conventional crystals grown by non-contaminating methods, as for example, the floating zone method, on the other hand, contain a disclocation density of approximately to 10 dislocations per square centimeter.

The oxygen-free, dislocation-free semiconductive crys tals grown in accord with the present invention may be of high purity substantially intrinsic semiconductive material for use in thermosensitive and photosensitive elements or may, on the other hand, contain substantial quantities of significant acceptor or donor activator impurities or both and may be utilized in the fabrication of asymmetrically conductive devices such as P-N junction rectifiers and transistors. Thus, for example, both a significant donor and a significant acceptor activator impurity may be included in disc so that upon withdrawal of ingot 16 therefrom a semiconductor ingot having a dominant and a non-dominant impurity therein is formed which ingot is suitable for the starting material for the local fusion technique of manufacturing semiconductor devices as disclosed and claimed in the copending application of Robert N. Hall, Serial No. 516,637, filed June 20, 1955, now US. Patent 2,822,308. Alternatively, the ingot may be utilized as a source of starting material for the fabrication of fused and diffused P-N junction rectifiers and transistors as set forth in the copending application of- Robert N. Hall, Serial No. 596,943, filed July 10, 1956. In both these instances, if the ingot is desired to possess N-type conductivity characteristics, the significant activator impurity added to disc 20 prior to growth of ingot 16 therefrom may comprise phosphorus, arsenic, or antimony. If, on the other hand, it is desired that the grown ingot possess P-type conductivity characteristics the significant activator impurity may comprise boron, aluminum, gallium, indium or mixtures thereof.

While the invention has been described with particula-rity hereinbefore, the following specific example is set forth for the further guidance of those skilled in the art. This specific example is exemplary only and is not to be construed in a limiting sense.

Example 1 A monocrystalline, cylindrical rod of silicon 1% inches long and /2 inch in diameter is out from one end to within inch of the remaining end with a plurality of longitudinal slots approximately 0.030 inch wide disecting the cross-sectional area of the rod into 60 sectors. A /2 inch diameter disc approximately inch thick is then mounted upon the sector support member and placed within thes supporting jig as illustrated in Fig. 1 of the drawing. A seed crystal of monocrystalline P-type silicon having a resistivity in excess of 50 ohm centimeters is cut into a needle approximately 1 inch long and 0.020 inch in diameter with the (1,0,0) plane perpendicular to the longitudinal axis of the needle. This seed crystal is inserted in the chuck on the crystal pulling mandril and the apparatus is closed and exhausted by means of the vacuum pump to a pressure of 0.01 micron. The exterior of vacuum enclosure 1 in the vicinity of the silicon disc is then heated with an acetylene torch until the silicon disc is heated to a temperature of approximately 500 C. as evidenced by a dull red glow. Electrical power is then supplied to induction heating coil 4 sufficient to raise the tempera ture of silicon disc to approximately 1420 C. causing it to melt and form a globule which is maintained in position on top of the slotted silicon support member by surface tension and levitation from the electromagnetic field. The mandril is then lowered until the seed crystal makes contact with the surface of the molten silicon and is observed to melt. The seed crystal is then with, drawn from the melt by a mechanically driven apparatus which withdraws the crystal at a constant rate of approximately 5 mm. per minute. This process is continued for approximately 10 minutes during which time a monocrystalline ingot of silicon weighing 3 grams was withdrawn therefrom. The crystal was approximately 2 /2 inches long and 7 inch in diameter. Wafers cut from the central portion of this crystal showed no detectable oxygen content under infrared absorption at 9 microns and no detectable dislocations.

While the invention has been set forth hereinbefore with respect to certain embodiments thereof, many modifications and changes will immediately occur to those skilled in the art. Accordingly, by the appended claims, I intend to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming oxygen-free, dislocationfree single crystals of semiconductor material which method comprises: supporting a first body of the semiconductive material capable of sustaining eddy currents entirely by a second body of the semiconductive material incapable of sustaining eddy currents; subjecting said first body of semiconductive material to a radio frequency electromagnetic field to causesaid first body to melt and to form a molten globule of semiconductive material by surface tension and electromagnetic levitation; inserting a monocrystalline seed crystal of the semiconductive material into the molten globule of semiconductive material, said seed crystal being very small in cross-sectional area as compared with the cross-sectional area of molten semiconductive material; and

growing a dislocation-free, oxygen-free monocrystalline ingot of semiconductive material from said molten globule by seed crystal withdrawal therefrom.

2. The method of claim 1 wherein the semiconductive material is silicon.

3. The method of forming oxygen-free, dislocationfree crystals of semiconductive material which method comprises: supporting a first body of the semiconductive material capable of sustaining eddy currents entirely upon a second body of the semiconductive material incapable of sustaining eddy currents; subjecting said first body of semiconductive material to a radio frequency electromagnetic field to cause said first body to melt and to form a surface-tension held globule atop said second body; inserting a monocrystalline seed crystal of the semiconductive material into the molten globule of semiconductive material; said seed crystal being very small in cross-sectional area as compared with the cross-sectional area of molten semiconductive material; and growing a dislocation-free, oxygen-free monocrystalline ingot of semiconductive material from said molten globule by 0 seed crystal withdrawal therefrom.

4. The me hod of claim 3 wherein the semiconductive material is silicon.

5. The method of forming oxygen-free, dislocationfree crystals of semiconductive material which method comprises: supporting a first body of the semiconductive material capable of sustaining eddy currents entirely upon a second body of the semiconductive material incapable of sustaining eddy currents; subjecting said first body of semiconductive material to a'radio frequency electromagnetic field to cause said first body to melt and to form a surface-tension-held globule atop said second body; inserting a monocrystalline seed crystal of 'the seed crystal into the molten globule of semiconductive material, said seed crystal having an extremely small of semiconductive material to be continuously replenished by controllably melting by heat conduction from said molten globule the support member by thermal conduction.

6. The method of claim wherein the semiconductor material is silicon.

7. The method of forming oxygen-free, dislocationfree single crystals of semiconductive material which method comprises: supporting a body of semiconductive material capable of sustaining eddy currents entirely upon a slotted cylindrical support member of the semiconductive material wherein slots pass entirely through said member in a radial direction, said slotted member being incapable of sustaining eddy currents over a substantial portion of its length; subjecting the first body of semiconductive material to a radio frequency electromagnetic field to cause the formation of a surface tension-held globule of molten semiconductive material atop the unmelted slotted support member; inserting a monocrystalline seed crystal of the semiconductive material into the molten globule of semiconductive material, said seed crystal being very small in cross-sectional area as compared with the cross-sectional area of the molten globule; and growing a dislocation-free, oxygen-free monocrystalline ingot of semiconductive material from said molten globule by seed crystal withdrawal therefrom.

8. The method of claim 7 wherein the semiconductive material is silicon.

9. The method of forming oxygen-free, dislocationfree single crystals of semiconductive material which method comprises: supporting first a body of semiconductive material capable of sustaining eddy currents entirely upon a slotted cylindrical support member of the semiconductive material wherein said slots pass entirely through said member in a radial direction, said support member being incapable of sustaining eddy currents over a substantial portion of its length; subjecting the first body of semiconductive material to a radio-frequency electromagnetic field to cause the formation of a surface tension-held globule of molten semiconductive material atop the unmelted slotted support member; inserting a mono-crystalline seed crystal of the semiconductive material into the molten globule, said seed crystal having a cross-sectional area which is extremely small as compared with the cross-sectional area of the molten globule; and growing a monocrystalline ingot of the semiconductor material from the molten globule by seed crystal with drawal therefrom while controlling the electric power supplied to the electromagnetic field to cause the molten globule to be continuously replenished by controllably melting the support member by thermal conduction from said molten globule.

10. The method of claim 9 wherein the semiconductor material is silicon.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF FORMING OXYGEN-FREE, DISLOCATIONFREE SINGLE CRYSTALS OF SEMICONDUCTOR MATERIAL WHICH METHOD COMPRISES: SUPPORTING A FIRST BODY OF THE SEMICONDUCTIVE MATERIAL CAPABLE OF SUSTAINING EDDY CURRENTS ENTIRELY BY A SECOND BODY OF THE SEMICONDUCTIVE MATERIAL INCAPABLE OF SUSTAINING EDDY CURRENTS SUBJECTING SAID FIRST BODY OF SEMICONDUCTIVE MATERIAL TO A RATIO FREQUENCY ELECTROMAGNETIC FIELD TO CAUSE SAID FIRST BODY TO MELT AND TO FORM A MOLTEN GLOBULE OF SEMICONDUCTIVE MATERIAL BY SURFACE TENISON AND ELECTROMAGNETIC LEVITATION, INSERTING A MONOCRYSTALLINE SEED CRYSTAL OF THE SEMICONDUCTIVE MATERIAL INTO THE MOLTEN GLOBULE OF SEMICONDUCTIVE MATERIAL, SAID SEED CRYSTAL BEING VERY SMALL IN CROSS-SECTIONAL AREA AS COMPARED WITH THE CROSS-SECTIONAL AREA OF MOLTEN SEMICONDUCTIVE MATERIAL, AND GROWING A DISLOCATION-FREE, OXYGEN-FREE MONOCRYSTALLINE INGOT OF SEMICONDUCTIVE MATERIAL FROM SAID MOLTEN GLOBULE BY SEED CRYSTAL WITHDRAWAL THEREFROM. 